1. Field of the Invention
The present invention relates to duplexer packages employing a surface-acoustic-wave band pass filter and, more particularly, to multilayer duplexer packages having phase matching circuits.
2. Description of the Related Art
In recent years, mobile communications apparatuses such as handy phones have been rapidly down-sized and, hence, the parts for use therein have been desired to have a reduced size and higher performance. In radio communications apparatuses, duplexers are used to prevent the interference of transmitted and received signals. While most of the duplexers comprise band pass filters or band filters utilizing dielectric or combinations thereof, those employing surface-acoustic-wave filters are now under research and development.
A duplexer comprising two surface-acoustic-wave band pass filter chips F1 and F2 should be designed so as to prevent the interference between the filter characteristics of these surface-acoustic-wave band pass filters. To this end, the surface-acoustic-wave band pass filters are each provided with a phase matching circuit.
In general, the aforesaid filter chips F1 and F2 are designed to each have an impedance close to a characteristic impedance (typically 50 .OMEGA.) of the entire filter circuit around the center band pass frequency thereof, and have an impedance much higher than the characteristic impedance in frequency ranges out of the pass band. However, it is difficult to prevent one filter chip from interfering with the filtering characteristics of the other filter chip in a pass band thereof due to the influence of resistance present in the circuit pattern. For this reason, the phase matching circuit is needed. The constant of the phase matching circuit is determined by respective center band pass frequencies f1 and f2 of these two filter chips and the difference therebetween.
Conventionally proposed phase matching circuits employ L (inductance) element and C (capacitor) element or lines serving as L and C components.
For example, Japanese Unexamined Patent Publications HEI 5(1993)-167388 and HEI 5(1993)-167389 disclose duplexers having phase matching circuits formed of metal strip lines on a glass-epoxy resin substrate or ceramic substrate. Otherwise, there is known a duplexer package comprising a multilayer ceramic package having two filter chips and phase matching circuits accommodated therein.
In FIGS. 10(a) and 10(b), there is shown one example of a conventional duplexer comprising this type of multilayer ceramic package. The duplexer shown includes filter chips 7 and 8 mounted on a multilayer ceramic package 6 having a plurality of ground layers GND, phase matching circuits 1 and 2, ground terminals 3, filter signal terminals 4 and common signal terminals 5, wherein the filter chips 7 and 8 are each connected to the terminals 3, 4 and 5 by means of wires 9.
As shown, the phase matching circuits 1 and 2 are disposed below the filter chip layer and each is sandwiched between the GND layers in the package. The phase matching circuits 1 and 2 are typically formed of strip lines, and the characteristic impedances thereof are respectively adjusted equal to the characteristic impedance of an external circuit connected to the common signal terminals 5, so that the circuit loss can be reduced.
However, the characteristic impedances of these two filter chips having different center pass band frequencies in the duplexer vary depending on signal frequencies to be used. For example, the characteristic impedance of each of the filter chips is substantially the same as that of an external circuit connected thereto in a pass band, and is much lower or higher than that of the external circuit in a stop band. If these filter chips are to be used for the construction of a duplexer, an attempt must be made to prevent degradation in the characteristics of the filter chips and the external circuits for a reduced circuit loss. For this reason, it is ideal that the characteristic impedance of one filter chip is infinitely great and the reflection coefficient thereof is generally 1 in the pass band of the other filter chip.
To attain such ideal characteristics, phase matching circuits are required. To meet this requirement, the formation of the aforesaid strip lines has been proposed. In this case, the resistance increases proportionally to the strip line length. The increase in resistance may cause a signal transmission loss and increase in the distribution constant of stray capacity to occur. The increase in stray capacity influences the phase circuit constant and the like, and such influence increases as the frequency of signals to be used grows higher. That is, the branching characteristics of transmitted and received signals are degraded, resulting in a transmission loss of the signals.
If the package is made of a material having a high dielectric constant, the layers have to be bonded to each other at a high temperature. This means that the strip lines should be formed of a high melting-point metal. Therefore, the aforesaid filtering characteristics may be degraded to a larger extent.